1. Field of the Invention
The present invention relates generally to a power amplifier, and is directed more particularly to a power amplifier in which a power supply voltage is changed over.
2. Description of the Prior Art
In the art, there has been proposed such a power amplifier in which the power supply voltage is changed over in response to the amplitude of an input signal or output signal. This kind of the power amplifier has been practically used in various fields since the power amplifier is less in heat generation, high in efficiency and economical. However, up to now there has been proposed no such power amplifier high in efficiency in which an output current flowing through a load is parallely changed over in response to the amplitude of an output signal. That is, in a conventional high efficiency amplifier, output transistors are connected in series to a power supply source so that an excess current flows through all of the output transistors when an input signal is large. Therefore, it is necessary to employ transistors which will withstand a large current as all the output transistors, which results high in cost.